Left Join in Django - python-3.x

Question

I have the following models:
class Patient(models.Model):
patient_first_name = models.CharField(max_length=50)
patient_last_name = models.CharField(max_length=50)
patient_name = models.CharField(max_length=100)
patient_email = models.EmailField(max_length=100)
gender = models.CharField(max_length=50)
class PatientMedicalRecord(models.Model):
patient = models.ForeignKey(Patient)
mrn = models.CharField(max_length=50, unique=True)
patient_height = models.IntegerField(blank=True, null=True)
patient_weight = models.IntegerField(blank=True, null=True)
age_risk = models.BooleanField(default=False)
I want to query on patient table for getting all the patient. also i need MRN column value from PatientMedicalRecord table which contain record for particular patient if exists.
How can i do this with djnago ORM?
Following are sql query gives me perfect result.
SELECT a.id,--remaining field, b.mrn FROM patient as a LEFT JOIN patient_medical_record as b ON a.id=b.patient_id;
How can i do this with django annotate ?

Answer 1

You can fetch related objects using the object_set. In your example, here is how you would do it:
patient = Patient.objects.get(pk=1) # You can use any attribute to get the Patient object
patient_medical_records = patient.patientmedicalrecord_set.all()
patient_mrns = []
for record in patient_medical_records:
patient_mrns.append(record.mrn)
You can also defined a related_name property in your model for the relationship to query relationships with. For example:
class PatientMedicalRecord(models.Model):
patient = models.ForeignKey(Patient, on_delete=models.CASCADE, related_name='patient_records')
Then you would query it like this:
patient = Patient.objects.get(pk=1)
patient_medical_records = patient.patient_records.all()